Hypoplastic Left Heart Syndrome remains the leading cause of death of all congenital heart defects in the first year of life. Traditionally, three staged surgical palliations are performed during the first 2-3 years of life, requiring profound hypothermic circulatory arrest and cardiopulmonary bypass. Neurodevelopmental outcomes in this cohort of patients remain suboptimal. The Hybrid Stage I procedure offers an alternative approach for the first staged palliation. The Hybrid procedure does not require cardiopulmonary bypass;therefore the neonate is not exposed to all the risks of circulatory arrest and cardiopulmonary bypass with potential for neurologic injury. After the Hybrid procedure, cerebral perfusion is dependent upon the blood flow through the stented patent ductus arteriosus and retrograde aortic arch flow to the carotid arteries, similar to fetal life, for approximately 6 months of age when the next staged repair is performed. It remains unknown if the retrograde blood flow from the aortic arch to the brain is adequate for normal growth and development. The long-term objective of this research proposal is to test the hypothesis that a correlation exists between cerebral blood flow velocity and neurodevelopmental outcomes in hypoplastic left heart syndrome babies who have undergone Hybrid Stage I palliation. HLHS patients, along with normal age- matched controls, will be recruited for this study. Transcranial Doppler will be performed to measure the middle and anterior cerebral artery blood flow after birth, and at 2, 4, and 6 months of age in both groups. Additionally, infant developmental assessment will be performed at the same time intervals using the Test of Infant Motor Performance at birth and 2 and 4 months, as well as the Bayley Scale of Infant and Toddler Development, 3rd edition, performed at 6 months of age. This will be compared to norm-referenced age- matched controls. Although the sample of HLHS patients is expected to be low, this study will include infants of both gender, as well as minorities. The transcranial Doppler findings of this study may have a profound impact on neurocognitive developmental outcomes which may have a tremendous long-term impact on patients, families, and society. This study is aligned with the mission of the NIH in biomedical and behavioral research with neurodevelopmental outcomes, coupled with innovation of current technology with transcranial Doppler, to assess cerebral perfusion in this unique cohort of HLHS patients after Hybrid Stage I palliation.